Electrolysers of the filter-press type comprise a so-called electrolyser module and a number of peripheric components such as degassing chambers, a water supply unit and possibly a transformer/rectifier and the necessary piping to connect the various parts of the electrolyser. An electrolyser module comprises a series of stacked electrolysis chambers, alternately cathodic and anodic. Each chamber holds either one or more anodes or cathodes. The electrolysis chambers are separated from one another by selectively permeable membranes or diaphragms. A combination of a cathodic and an anodic chamber form an electrolysis unit cell. In each chamber, the electrodes are mounted vertically, preferably in close contact with the membranes. This can be accomplished, for example, by pressing the membrane between the electrodes. In preferred executions, perforated electrodes are used. Membranes that are particularly suited for this purpose have been described in EP-A-0 232 923.
The electrolysis chambers are held together by a frame, which forms the outer wall of the electrolyser and may take a variety of shapes, e.g. polygonal such as square, rectangular, or circular. In the last-mentioned instance the frame is in fact ring or cylinder shaped. The separating membrane is fixed to the frame together with the electrodes. The frames are made of a material that is inert to the electrolyte and can be made for example of metal, covered by a suitable non-conducting layer, or can be made of a suitable inert non-conducting material such as a synthetic, preferably polymeric material. A frame made of metal covered by a flexible vulcanizable material is known from WO-97/00979.
A single holding frame from a unit cell has a central opening, which forms the interior of an electrolysis chamber. Typically this opening is circular although it may also have other shapes. The size and shape of this central opening as well as any other openings is the same in all holding frames of the electrolyser module. A holding frame usually also has one or two circular openings for the electrolyte (which may be an aqueous potassium hydroxide solution) and one opening for the produced gas to escape. The frames are arranged in such manner that the openings are adjacent to one another thus forming a conduit.
The active cell area is defined as the area, which is exposed to the electrolyte liquid held in the anodic or cathodic chamber. The active cell area is determined by the size of the central opening of the holding frames.
In each electrolysis chamber the electrodes have to be linked by a suitable conductor. In particular embodiments this can be accomplished by contacting the electrode with a metal woven sheet, which in turn is contacted with a metal plate that is mounted in the frame, said plate being referred to in the art as the bipolar plate.
The electrolyser module therefore is composed of unit cells put electrically in series and hydraulically in parallel through the above mentioned frames and are tied together using tie-rods.
The gas that is generated is led to a degassing chamber, which mainly functions as a gas/liquid separator, for example based on the decantation principle, which is mounted on top of the electrolyser module. The degassing chambers typically are cylinder type vessels that are physically separated from the cell stack, where the gas is collected and separated off the electrolyte entrained with the gas. They can be positioned in parallel as well as perpendicular to the electrolyser module.
The de-gassed electrolyte is recycled back to the electrolysis chambers and the gas that is collected can be pressurized and stored in suitable pressure tanks.
The electrolysis chambers may be connected to outer tubes leading to or coming from the de-gassing chamber for respectively the circulation of the generated gasses or of electrolyte. Or the electrolysis chamber holding frames may have suitable conduits at their top side and at their bottom side. The top conduit is meant to evacuate the gas-electrolyte mixture that is generated during electrolysis and the bottom conduit allows the entrance of water or electrolyte.
The gas-electrolyte mixture that is generated is pumped through the upper conduit to a tube leading to the de-gassing unit from which the de-gassed electrolyte is pumped back to the bottom conduit in the frame from which it enters the electrolysis chambers.
Filter-press electrolysers have been described in EP-A-137,836 and in EP-A-56,759.
U.S. Pat. No. 5,139,635 for example describes gas electrolysers of the filter-press type comprising a vertical stack of electrolysis chambers connected to a degassing chamber.
EP 1133586 describes a high pressure electrolyser module having frames of special design that allow the electrolyser module to function at high pressure thus avoiding an additional gas-compression step so that the formed gas can be directly stored.
Hence the art-known gas electrolysers of the filter-press type are quite large and complex arrangements involving several peripheral items such as pumps, tanks and piping, and therefore comprise a multitude of moving parts, requiring supervision, checking and maintenance. Thus, a more simplified and compact arrangement with no or fewer moving parts would be a desirable goal to achieve in that it would require almost no or limited maintenance.
The compact electrolysers according to the present invention are aimed at avoiding moving parts and at the same time allowing the elimination of peripheral equipment resulting in a more simplified arrangement of the electrolyser, requiring less supervision and maintenance.
In the present invention, one holding frame includes the gas/liquid separator as well as the active unit cell area. The latter offers not only great economic advantage with respect to reduced manufacturing as well as material costs, but moreover results in a substantially more compact overall electrolysis module.